The present invention relates to an x-ray focusing system. More specifically the present invention relates to a modular system of lenses designed to focus high energy x-rays.
There are various applications which may benefit from the use of focused x-rays. The following background and description illustrate applications of the present invention which are merely exemplary in nature and are in no way intended to limit the invention or its uses. Moreover, the following description, while depicting an x-ray lens system designed to be used in medical applications and crystal diffraction applications, is intended to adequately teach one skilled in the art to make and use the present invention in a variety of x-ray applications, including, but not limited to x-ray structural analysis and x-ray spectroscopy.
Presently, medical applications such as radiotherapy use collimated x-rays for the destruction of malignant tissue. Radiotherapy is one of the major methods, sometimes the only method, in treating some types of cancers such as brain tumors. Linear accelerator systems generating x-rays have been widely used in radiotherapy in the destruction of such malignancies. Linear accelerator systems employed in radiotherapy generally use a multi-leaf collimator to create a shaped beam of x-rays. The shaped x-ray beam intensity has a flux density consistent throughout its extent. The energy range of x-rays generated by such a system usually reach into the MeV range to be effective. To destroy a tumor the linear accelerator system must be continually directed at and rotated about the targeted malignant tissue. The high energy (MeV) of linear accelerator systems and their collimated rays expose a large amount of healthy tissue surrounding a tumor to a potentially damaging concentration of x-rays in the MeV range. The focused x-ray beam of the present invention provides a high brightness focal spot of lower energy x-rays which is used to treat a target in an accurate controlled fashion, as well as treat the target at an early stage. Lower energy x-rays have quicker fall-off behind the target and therefore reduce tissue damage to some sensitive organs which may be exposed to x-rays.
A system utilizing the x-ray focusing properties of the present invention can achieve the same results with reduced damage to collateral tissue and an energy use in the 40 KeV-100 KeV range. The advantages of using this focusing system include: reduced exposure and damage of healthy body tissue to x-rays, the x-rays in the KeV range can be focused directly at a malignancy with decreasing radiation intensity surrounding the x-ray focal point/treatment area, eliminating damage to sensitive organs proximate the target, the energy of the x-rays can be set above the absorption edge of certain materials such as drugs that are delivered to the tumor; the treatment of very small tumors can be done in a more precise manner; and there is an overall lower cost of the present invention as compared with previous linear accelerator systems.
The x-ray focusing properties of the present invention may also be used in the study of crystal structures. A common method used to study crystal structures is x-ray diffraction. The method is based on illuminating a sample crystal with a beam of x-rays. A portion of the x-ray beam is not able to travel directly through the sample crystal, rather some rays are deflected or diffracted and emerge from the sample at varying angles. The incident x-rays make their way along the spaces between the atoms of the crystal or are deflected by the atoms. A sensor is used which detects the x-ray diffraction pattern generated by the x-rays as they emerge from the sample crystal. This diffraction pattern corresponds to the atomic structural arrangement of the crystal. Such a system is known in the art as an x-ray diffractometer. The focusing properties of the present invention can improve the flux concentration on a sample crystal leading to improved diffraction patterns.
Many devices can be used to focus, and/or reflect x-rays such as total reflection mirrors, bent single crystals, graded multi-layer devices, and mosaic crystals. The main purpose of these devices is to gather Y-ray flux produced by an x-ray generator and direct it to a desired area. There are three main factors which determine the flux strength of a reflecting and focusing device: reflection angle, reflectivity, and rocking curve width. Reflection angle is the angle at which x-rays are reflected from the surface of the reflection surface, reflectivity is the amount of energy returned from a surface after x-rays are incident upon that surface, and rocking curve width is the ability to collect and reflect energy over a particular incident range.
The total reflection mirror has the smallest reflection angle of all the previously mentioned devices, which results in the smallest capture angle and in turn, small throughput, although its reflectivity approaches 100%. The total reflection mirror will also reflect the desired and undesired x-ray wavelengths. In medical applications, these undesired x-ray wavelengths could potentially cause skin damage to a person undergoing treatment.
Bent single crystals have a large reflection angle and high peak reflectivity but a very narrow bandpass limiting the gathered flux to a small amount.
Multi-layered x-ray reflectors have a fairly wide rocking curve width and high peak reflectivity. The reflection angle is also larger than a total reflection mirror. The reflectivity and rocking curve width will drop when smaller d-layer spacing is used to achieve larger reflection angles. For high energy x-rays, such as in the multiple 10 KeV range, the x-ray focusing efficiency of the multi-layer reflector is not satisfactory.
Mosaic crystals consist of numerous tiny independent crystal regions which are nearly parallel but not quite parallel to one another. Mosaic crystals such as a graphite crystal have high reflectivity, a large reflection angle, and therefore a large capture angle. Mosaic crystals also have a large rocking curve width due to their mosaic structure. All of these factors make the mosaic crystal an attractive choice for reflecting and directing high energy x-rays. The focusing lenses of the present invention are composed of mosaic crystals such as a graphite crystal.
The present invention is a modular system of lenses used for focusing x-rays. The lenses are operated using the principles of Bragg reflection and Laue diffraction. The ideal crystal surfaces and crystal planes of these lenses follow the Johansson scheme. In practice, cylindrical, conical, even polygonal surfaces can be used for approaching the focusing scheme. The lenses using Bragg diffraction deliver a beam of narrow frequency band (substantially monochromatic) x-rays, while the lenses using Laue diffraction deliver a controlled wide frequency band of x-rays. Many lenses can be designed to have the same source-focal point distance. Each of the lenses has different source-lens and lens-focus distances, depending on the requirements of focal spot size, working distance (front end of the lens to focal point), and flux, different lenses or combinations of several lenses can be used. This modularity creates a simple yet flexible scheme for varying intensity, focal spot size, and working distance.
The lenses utilizing Bragg reflection use mosaic graphite crystal on their inner surfaces arranged in a cylindrical configuration. The lenses are formed by the bending of graphite layers or alternatively the direct growth of graphite on a lens housing. Graphite was chosen as preferred mosaic crystal in the Bragg lenses because of its superior reflective properties. The Laue lenses utilizing a Laue transmission scheme are similarly comprised of mosaic graphite crystal, but the x-rays are transmitted and diffracted through the crystal volume rather than being reflected only from the incident surface of the mosaic graphite crystal.
For many applications, different focal spot sizes and different intensities are needed for varying flux density requirements. These requirements can be met by the use of supplementary Bragg reflective x-ray lenses with internal spherical, cylindrical, conical, parabolic, ellipsoid or other conic type configuration, but are not limited to such configurations. These supplementary x-ray lenses can be used to collect x-rays and focus them at varying focal lengths and create varying focal point areas and intensities. This modularity creates a simple yet flexible scheme of varying the intensity, focal length, and focal point area of an x-ray beam.